Thrombotic cardiovascular diseases including venous thromboembolism (VTE), deep vein thrombosis (DVT), myocardial infarction, and ischemic stroke, remain leading causes of death and disability in the U.S. Although effective antithrombotic agents are available, these drugs inadvertently target vital hemostatic molecular mechanisms and can produce severe dose-limiting hemorrhagic toxicity, thereby limiting their use in some patients. Consequently, there is a significant and urgent unmet medical need for safer antithrombotic treatment alternatives. The proposed research will continue the development of an antithrombotic antibody against coagulation factor XI (AXIMAB 1A6) for the safe prevention and treatment of acute venous thrombosis. We have reached our Phase I milestones: 1) Establishing a minimum saturating dose of 1A6 (0.1mg/kg, iv) that is antithrombotic in baboons, 2) Showing that 1A6 has significant antithrombotic effects that are comparable to a high interventional dose of the low-molecular-weight heparin enoxaparin (Lovenox(R), 1mg/kg, iv), and 3) Verifying that the antithrombotic dose of AXIMAB 1A6 produces no measurable hemostatic impairment, in contrast to enoxaparin, which increases bleeding in the baboons. During Phase I we also discovered that 1A6 is more effective than our other prototype AXIMAB antibody 14E11 at limiting thrombus development under arterial shear flow conditions and during tissue factor initiated venous-type thrombosis. The difference in efficacy is likely related to distinct FXI molecular pathways that are targeted by the two antibodies. We therefore propose to continue the commercial development of AXIMAB 1A6 as our hemostatically safe and effective antithrombotic drug candidate. We have successfully advanced AXIMAB product development and are now prepared to further move AXIMAB towards clinical trials for the short-term prevention and treatment of acute DVT and VTE. The Specific Aims for this Phase II project that will be necessary to support the development of AXIMAB 1A6 through an investigational new drug (IND) application are to: 1) Evaluate the activity and efficacy of recombinant humanized AXIMAB 1A6, 2) Characterize manufactured GMP-grade humanized AXIMAB 1A6 formulations, and 3) Determine the dose-limiting toxicity of GMP-grade humanized AXIMAB 1A6 in preclinical studies. The AXIMAB approach represents a fundamentally new anticoagulation concept since the contribution of FXI to pathological coagulation appears to far outweigh its role in normal hemostasis. Thus, AXIMAB could represent an effective antithrombotic strategy that is exceptionally safe. Success of this Phase II research and achievement of our critical milestones will lead directly into the next stage of product development that will consist of IND preparation and filing, followed by the initiation of phase 1 clinical studies to safely prevent and treat acute venous thrombosis and thromboembolism.